Make "EditPro" controller work with diff updates

Summary:
Ref T2222. Make the "EditPro" controller accommodate diff updates, and support the transaction type. This one is pretty straightforward.

Also make revisionPHID in the comments table nullable to fix the "Edit" action.

Test Plan:

• Created new revision.
• Updated revision.
• Tried to do some invalid stuff.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2222

Differential Revision: https://secure.phabricator.com/D8376